Ignition systems for turbomachine engines use various technologies, and particularly ignition by spark plug, which is used for example to start gas turbines in aeronautics and industry.
In principal, turbomachines consist of three elements: the compressor, the combustion chamber, and the turbine.
The spark plug is situated inside the combustion chamber. This spark plug serves to transmit a quantity of energy delivered via a power supply, and creates a spark at one end thereof. This spark is caused by the breakdown of a gas-phase medium that surrounds two electrodes on the spark plug.
Two types of spark plug are currently in use: high voltage spark plugs (20 kV, a starting voltage which rises significantly under high pressures) inside the combustion chamber, and low voltage spark plugs including a semiconductor ceramic set between the two electrodes of the spark plug that cause the gas-phase medium to ionize and separating them regardless of the pressure inside the combustion chamber.
The advantage of spark plugs that include a semiconductor ceramic is that the voltage required to produce the spark is constant and unaffected by pressure.
The spark plugs must be able to withstand extreme operating conditions, particularly when they are used in aeronautical applications.
During operation, spark plugs containing a semiconductor ceramic are exposed to a number of different stresses.
One of these stresses is related to the strong thermal shocks that the spark plug must withstand due to the extremely rapid temperature rise inside the chamber (from 0° C. to 900° C. in one minute).
Another stress is related to the transition from an atmosphere that is very rich, and thus highly reductive, during take-off, to an atmosphere that is very lean, and thus highly oxidising, during flight.
Other stresses are related to the vibration of the engine, saline mist, humidity, and even freezing.
Thus it is in these extreme conditions that an electrical arc must be sustained between the two electrodes of the spark plug to enable the fuel to be ignited or reignited inside the combustion chamber.
The semiconductor ceramic material in modern spark plugs consists of two phases in the following proportions: 40% Sialon and 60% SiC. The ceramic in this type of spark plug has a high porosity, in the order of 15%. This porosity is distributed over the internal and external surfaces and extends to a depth of about a quarter of the thickness on each ceramic face. The pores are the sites of micro-discharges which enable the gas-phase media to be broken down in these microcavities.
SiC is a semiconductor with a wide bandgap and a large breakdown field (2 MV·cm−1) combined with good thermal conductivity, which renders it a material of choice for dissipating strong electric forces.
For its part, Sialon manifests good resistance to high temperature. It is a hard, wear-resistant material. It is chemically inert and unaffected by thermal shocks, it also has a low coefficient of expansion and is resistant to oxidation.
However, the main drawback associated with spark plugs that include a semiconductor ceramic as described above consists in their short service life and poor reliability.
Indeed, the semiconductor material is degraded quickly by the combination of extreme thermal shocks, maximum temperatures that are becoming higher and higher, and the oxidising atmosphere.
SiC is oxidised at temperatures above 600° C., which distorts the electrical properties of the material and significantly impairs its electrical conductivity.
The porosity of the ceramic renders it weaker and makes it more susceptible to premature wear.
At the same time, efforts to improve the output of thermal engines and reduce the emission of pollutants that such engines discharge into the atmosphere necessitate further increasing the temperature inside the combustion chambers.
As the temperature and pressure conditions in the combustion chambers are becoming more and more extreme, spark plugs that contain semiconductor ceramic according to the current state of the art are becoming unusable, and are thus reaching the limit of the technological capabilities.
One object of the present invention is to suggest a new semiconductor ceramic that serves to mitigate the disadvantages of SiC— and Sialon-based semiconductor ceramics.